FrogReaver
The most respectful and polite poster ever
I'm perhaps misunderstanding.
I'm just suggesting the target number is derived from something to do with the orcs statblock rather than a number selected for whatever reasons the DM feels is appropriate. Success or failure on said check determines what happens. I'd operate on the basis that failure means the first group of orcs spots the sneaking character, however that isn't necessarily how it should go with my suggestion.
I'm suggesting that basing it on an NPC statblock is incorrect. When you aren't being challenged by an NPC but instead by the scenario then the DC is set for the scenario.
On a success we do the same thing you're doing, you get to the next stage of whatever you want to be happening. On failure at least one orc has spotted you. What happens? No where that happens is going to depend on how as a group we want to handle that.
Then you are using scenario based DC, but incorrectly equating it to the orc passive perception every time. The scenario presented could be much harder than an Orc's passive perception dc, in which case using the passive perception of an orc for the DC makes success easier than it ought to be. That's the problem with how you specifically are doing it.
I'm curious to get more detail about how you would handle your process and where it differs. Lets assume in your scenario we have a target number of 20. Our erstwhile PC, lets call them Pat Chaperone, get a total of 19 on their relevant check. What happens? Clearly Pat has failed to be sneaky, but what would you do to determine what happens? What would be different if Pat instead got a total of say 4 instead?
Sure. There's no set answer to what to do. It's a failure and obviously he's spotted. I might have it be by a random patrol, in which case he gets chased by them without the whole camp being alerted. I might have it be by a single guard in the tower, who leaves his post to discretely tell his superiors about the approacher without ringing alarm bells so as to not spook him away (goal of capturing PC). Or maybe 3-4 see him and one of them sounds an alarm.
In terms of a close success and low roll failure, there doesn't have to be a difference. If I wanted to give the PC an extra chance on the close success, maybe the guard thought he seen something but wasn't sure and is staring out there. I'd probably give the rogue a chance to notice this, which would allow him an opportunity to back off or to take cover and wait for the guard to be distracted again, or to be oblivious and continue on till he is surely spotted.